Net ecosystem exchange of extensive green roofs: the role of coupled energy, carbon, and water fluxes quantified by long-term micrometeorological observations

Vegetated roofs (i.e. green roofs, GRs) have been emerging as a promising nature-based solution in urban environments to mitigate climate change impacts, such as heat waves, urban flooding, and increased greenhouse gas emissions. Green roofs were shown to provide various ecosystem services, such as carbon sequestration from the urban atmosphere. The present study leverages a 9-year, long-term time series of continuous, integrated flux measurements on a large, extensive GR in Berlin, Germany, using the eddy-covariance technique. We investigate, (1) whether the GR is a net annual sink for carbon, (2) if the sink intensity remains stable over the whole study period, and (3) the coupling between carbon, water, and energy fluxes at different temporal scales to determine their role in shaping the net ecosystem exchange (NEE) of the GR ecosystem. The extensive GR was a moderate carbon sink with an average annual NEE of −92, ranging from −154 to +8 g C m−2 in the study period from 2015–2023. In the final two years, 2022 and 2023, the annual NEE shifted toward net carbon release, coinciding with an abrupt increase in substrate organic carbon, which has been linked to external input of carbon to the GR system. During the study period, the roof retained 51 % of precipitation, with a strong coupling between soil moisture, evapotranspiration, sensible heat flux, and carbon flux. Low substrate water content (<≈0.05 m3 m−3) reduced evaporative cooling and suppressed carbon uptake during the summer. The findings demonstrate the importance of integrated flux monitoring and emphasise the multifaceted environmental benefits of extensive GRs while also pointing to their structural constraints.